Ferromagnetic materials experience a dimensional change when they are subjected to a magnetic field. This phenomenon is called magnetostriction and is attributed to the rotations of small magnetic domains in the materials which are randomly oriented when the material is not exposed to a magnetic field. These magnetostrictive effects, including the Joule effect (change in length when ferromagnetic rod is placed in longitudinal field) and the Villari effect (change in magnetic condition when a magnetized ferromagnetic rod is subjected to longitudinal stress), can be used for converting electrical power to mechanical power and vice versa. Magnetostrictive smart materials can be used in sensor applications, principally because of the possibility to sense the control signal from a distance and without wires, which is not in the case for piezo-electric ceramics. Examples of the use of ferromagnetic materials include sensors (U.S. Pat. Nos. 4,414,510 and 5,442,966), transducers (U.S. Pat. No. 3,753,058), and vibrators (U.S. Pat. No. 4,151,432).
Recently metal oxides and their nanoparticles has been the subject of much interest because of their unusual optical, electronic and magnetic properties, which often differ from that of the bulk. Due to the high surface area to volume ratio, the fine magnetic particles show different properties when compared to the bulk.
In 1972, Clark and Belson discovered giant magnetostriction in alloys of terbium, dysprosium and Iron (Terfenol-D). It is a commercially available magnetostrictive material which is an alloy containing the rare earth elements terbium and dysprosium, showing high magnetostrictive strains around 1500 ppm at a relatively low magnetic field. However, this class of mechano-magnetic materials has many disadvantages which need to be carefully considered. These include the technological challenges of delivering controlled magnetic field to magnetostrictive actuator, apart from the high cost, high brittleness, requirement of single crystals, etc. Furthermore the material generates much greater response when it is subjected to the compressive loads, and the power requirement for this class of actuators is greater than those for piezoelectric materials.
It has been shown in U.S. Pat. No. 6,093,337 that magnetostrictive materials based on cobalt ferrite can be effective for stress sensing applications and in automobile industries as an alternative to the existing power steering systems with enhanced fuel efficiency. Metal bonded polycrystalline cobalt ferrite composite is shown in U.S. Pat. No. 6,352,649 to be better than polycrystalline Terfenol-based composites in terms of the higher piezomagnetic coefficient, corrosion resistance, magnetostriction and improved magnetomechanical response. These prior arts disclose the use of ferrites as magnetostrictive materials, giving high magnetostriction as well as having economic advantage over most commonly used Terfenol-D. Also, these composites show linear magnetostrictive strains of magnitude up to 225 ppm with a rate of change of magnetostriction with applied field (dλ/dH)max of 1.3×10−9 A−1 m under no external load. They also show good corrosion resistance as well as mechanical properties, apart from its low cost. But these metal bonded ferrites are very much susceptible to magnetomechanical hysteresis.
Improvements in both magnetostriction and strain derivative of polycrystalline cobalt ferrite have been reported (D. C. Jiles et al. IEEE Trans. Magn. 41, 10 (2005)) as a result of magnetic annealing. Results show that annealing cobalt ferrite at 300° C. in air for 36 hours under a dc field of 318 kA/m induced a uniaxial anisotropy with the easy axis being along the annealing field direction. Under hard axis applied fields, the maximum magnetostriction measured along the hard axis at room temperature increased in magnitude from 200 ppm to 250 ppm after annealing, strain derivative also increased from 1.5×10−9 A/m to 3.9×10−9 A/m. This result shows that magnetic annealing provides an alternative means other than chemical substitution for enhancing magnetomechanical properties of cobalt ferrite, for magnetoelastic stress or torque sensing applications. Paulson et al. have studied (U.S. Pat. No. 7,326,360) the effect of substitution of Fe by Mn in CoFe2O4 and showed that the incorporation of Mn leads to a decrease in the magnetostriction value from 200 ppm to 50 ppm and Curie temperature whereas the substitution enhances the possibility of reducing the magnetomechanical hysteresis. From the application point of view of sensing devices, it is desired that the materials should exhibit no such hysteresis.
Article titled “Magnetoelectric nano-Fe3O4/CoFe2//PbZr0.53Ti0.47O3 Composite” by Shenqiang Ren et. al in Applied Physics Letters, Volume 92, Issue 8, 16 Jan. 2008, Pages 83502-83504, relates to magnetoelectric hybrid device composed of a nano-particulate magnetostrictive iron oxide-cobalt ferrite film on a piezoelectric PZT crystal serving as both substrate and straining medium. Nano-Fe3O4/CoFe2O4 particles, ranging from 5 nm to 42 nm, were prepared using a variation of the sol-gel method.
Article titled “Effect of Sintering Conditions and Microstructure on the Magnetostrictive Properties of Cobalt Ferrite” by Shekhar D. Bhame et. al. in Journal of the American Ceramic Society, Volume 91, Issue 6, 18 Dec. 2008, Pages 1976-1980, investigates the effects of sintering temperature and dwell time on the magnetostrictive properties of polycrystalline cobalt ferrite synthesized by the conventional ceramic method. The highest value of magnetostrictive strain is observed for the samples sintered at 1100° C.
An article titled “Low-temperature auto-combustion synthesis and magnetic properties of cobalt ferrite nanopowder” by Shun Hua Xiao et. al. in Materials Chemistry and Physics Volume 106, Issue 1, 15 Nov. 2007, Pages 82-87, discloses an auto-combustion method by which Cobalt ferrite (CoFe2O4) nanopowder is obtained. But the synthetic process discloses is a thermally induced redox reaction with carboxyl group as reductant and NO3−1 ions as oxidant. The article further states that both the saturation magnetization (Ms) and the remanent magnetization (Mr) are highly dependent upon the annealing temperature where highest coercivity (1373 Oe) is achieved for the sample annealed at 400° C.
An article titled “Enhanced magnetostrictive properties of CoFe2O4 synthesized by an autocombustion method” by the present inventor (S. D. Bhame et. al.) in Sensors and Actuators A: Physical, Volume 137, Issue 2, 18 Mar. 2007, Pages 256-261, compares the magnetostrictive properties of sintered cobalt ferrite derived from nanocrystalline powders synthesized by three different low-temperature methods (citrate, coprecipitation and autocombustion process) and the high-temperature ceramic method. Maximum magnetostriction value of 197 ppm is obtained for the sintered ferrite samples with average grain sizes of 8 μm obtained by the autocombustion process using glycine as a fuel in the ratio of 2 moles of glycine per mole of metal ion.
An article titled “Synthesis and magnetic properties of nanostructured spinel ferrites using a glycine-nitrate process” by Nobuyuki Kikukawa in Journal of Magnetism and Magnetic Materials, Volume 284, 18 Jun. 2004, Pages 206-214, discloses the preparation of zinc-substituted spinel-type ferrite fine particles of M1-xZnxFe2O4 (M=Mg, Mn, Co, Ni, Cu, (Li, Fe) x=0-1) obtained by glycine-nitrate process by varying the ratio of glycine to nitrate (G/N ratio). The said article discloses synthesis of mono-phase spinel-type ferrites with the G/N ratio less than about 0.5 for the systems including Mg, Mg—Zn, Co, Co—Zn, Ni, Ni—Zn, (Li, Fe), and (Li, Fe)—Zn, and the ratio between 0.3 and 0.5 for Mn, and Mn—Zn systems. Further, TEM describes that the product powder consisted of agglomerates of primary particles with a typical diameter of about 50 nm for a G/N ratio of around 0.4. Table 1 of said article, discloses saturation magnetization of 79 emu/g and coercive force of 885 Oe measured for CoFe2O4 at the magnetic field of 15 kOe and at room temperature.
Cobalt ferrite is known for its high magnetostriction and there have been many attempts to make sintered polycrystalline cobalt ferrite composites with high magnetostriction at very low magnetic fields. So far there have been no studies reported on the effect of initial particle size and the resulting microstructure of the sintered material on magnetostriction. It is important to understand the effect of processing conditions, especially during sintering, in the case of nanomaterials. From the application point of view it is necessary to have very large magnetostricitive strains at room temperature. Since cobalt ferrite is an ideal material for future magnetostrictive applications because of its low cost, easy processability, etc. It is the object of the present invention to design a material based on cobalt ferrite with high magnetostriction.
In view of controlling the morphology of the powders and microstructure of the sintered products to obtain high values of saturation magnetostriction as well as to tune the low-field magnetostrictive response of polycrystalline cobalt ferrite, the present inventor further directed his research to provide improvements of both magnetostriction level and strain derivative of sintered cobalt ferrite composite material derived from nanosized and micron sized powders of cobalt ferrite, which are effective for use as magnetostrictive sensors and actuators. The further subject of the invention is to provide an improved low temperature auto-combustion process for the preparation of nanocrystalline powders.